Welding of Dissimilar Metals

When two different metals are joined together by welding, it is called dissimilar metal welding.

For welding such a joint, certain factors should be considered and these are given below:

(i) Thermal expansion heating during welding creates compressive stresses in the metal and cooling creates tensile stresses. A danger is there that a metal with low tensile strength may fail during cooling, Proper design of joint and pre-heating before welding may help in this regard.

(ii) Galvanic corrosion may occur due to the dissimilar metals in contact.

(iii) Metallurgical stability includes consideration of (a) formation of alloys and properties of alloy phases in the metals being welded, and (b) dilution.

Dissimilar metals are joined by fusion welding processes such as shielded metal-arc welding, resistance welding, electron beam welding, ultrasonic welding, friction welding, and brazing.

As already discussed elsewhere, dilution is the reduction in the alloy contents of the weld deposit. This is checked in several ways. The joint faces of the base metals (dissimilar) can be buttered or overlaid with a weld deposit such that welding will be done later between the faces of deposited weld.

Use an austenitic filler metal between chromium steels and austenitic steels; carbon steel rod buttering when welding carbon and chromium bearing metals; copper alloy for joining copper-base metals to iron-base metals; nickel alloy rod to butter nickel alloy with any dissimilar metal and vanadium buttering in welding titanium to steel. For fusion welding of dissimilar metals, the technique of using pre-fabricated joining piece is often employed.

The pre-fabricated joint piece consists of short sections of dissimilar materials that are to be welded. These short sections are prepared (joined) in the workshop under controlled conditions and later used in making a joint at site. This way an opportunity is made available to join only the similar metals; example includes joining of aluminium with copper for manufacturing refrigerators.

Resistance welding sometimes gives better results, for example, joining of copper and aluminium by flash-butt welding, spot welding or projection welding. Solid-state welding processes are often used successfully and effectively for welding dissimilar metals. Brazing is the most practised process of welding dissimilar metals of any type and without any problem.

However, metals susceptible to intergranular penetration should be annealed before brazing. Zinc bearing silver solders and copper-zinc alloys are used for brazing copper alloys, all types of heat-resisting alloys, nickel alloys and tool steels.